In extracorporeal blood processing, blood is taken out of a human or animal subject, processed (e.g. treated) and then reintroduced into the subject by means of an extracorporeal blood flow circuit (“EC circuit”) which is part of a blood processing apparatus. Generally, the blood is circulated through the EC circuit by a blood pump. In certain types of extracorporeal blood processing, the EC circuit includes an access device for blood withdrawal (e.g. an arterial needle or catheter) and an access device for blood reintroduction (e.g. a venous needle or catheter), which are inserted into a dedicated blood vessel access (e.g. fistula or graft) on the subject. Such extracorporeal blood treatments include hemodialysis, hemodiafiltration, hemofiltration, plasmapheresis, bloodbanking, blood fraction separation (e.g. cells) of donor blood, apheresis, extracorporeal blood oxygenation, assisted blood circulation, extracorporeal liver support/dialysis, ultrafiltration, etc.
It is vital to minimize the risk for malfunctions in the EC circuit, since these may lead to a potentially life-threatening condition of the subject. Serious conditions may e.g. arise if the EC circuit is disrupted downstream of the blood pump, e.g. by a Venous Needle Dislodgement (VND) event, in which the venous needle comes loose from the blood vessel access. Such a disruption may cause the subject to be drained of blood within minutes. Much research has been devoted to preventing and detecting VND, e.g. by improving the attachment of the needle and the associated tubing to the patient, by installing dedicated external equipment for detecting leakage of blood at the blood vessel access, or by monitoring the pressure measured by a pressure sensor (“venous pressure sensor”) on the downstream side of the blood pump in the EC circuit. Conventionally, the pressure monitoring is carried out by comparing one or more measured static pressure levels with one or more threshold values. However, it may be difficult to set appropriate threshold values, since the static pressure in the EC blood circuit may vary between treatments, and also during a treatment, e.g. as a result of the subject moving. Further, if the venous needle comes loose and gets stuck in bed sheets or the subject's clothes, the measured static pressure level might not change enough to indicate the potentially dangerous situation. To overcome these drawbacks WO97/10013, US2005/0010118, WO2009/156174, WO2010/149726 and US2010/0234786 all propose various techniques for detecting a VND event by identifying an absence of heart or breathing pulses in a pressure signal from a pressure sensor on the downstream side of the blood pump in the EC circuit.
Even if VND is a serious intradialytic complication, it is much less common than cardiac arrest, also known as cardiopulmonary arrest or circulatory arrest, which is the cessation of normal circulation of the blood due to failure of the heart to contract effectively. In 1999, the article “Sudden and cardiac death rates in hemodialysis patients” by Bleyer et al, published in Kidney International, Vol. 55 (1999), pp. 1553-1559, reported on an increased sudden and cardiac death rate for hemodialysis patients in the US. This utterly severe intradialytic complication has later been reported to occur in about 7 out of 100 000 treatments, which is about 5-10 times more common that VND, see the article “Cardiac arrest and sudden death in dialysis units” by Karnik et al, published in Kidney Int. 2001 July; 60(1):350-7. Based on the fact that about 150 treatments are performed annually for each patient among a global total of 2 million patients, it can be assumed that about 20 000 incidents of cardiac arrest occur during ongoing dialysis worldwide each year. The outcome of a cardiac arrest event for a dialysis patient is generally very poor: 13% of the patients die in the clinic in connection with the treatment and 60% die within 48 hours, as reported by Karnik et al. As noted by Alpert in the article “Sudden cardiac arrest and sudden cardiac death on dialysis: Epidemiology, evaluation, treatment, and prevention”, published in Hemodial Int, 2011 October; 15 Suppl 1:822-9, sudden cardiac arrest is the most common cause of death in dialysis patients.
According to Sasson et al in the article “Predictors of survival from out-of-hospital cardiac arrest: a systematic review and meta-analysis”, published in Circ Cardiovasc Qual Outcomes 2010, 3:63-81, only 8% of all persons with cardiac arrest survive after being given cardio-pulmonary resuscitation (CPR). Incidents taking place in a clinical environment have a better outcome, with a survival rate of 22% of witnessed cardiac arrests, as reported by Peter et al in the article “Predictors of survival following in-hospital adult cardiopulmonary resuscitation”, published in CMAJ 2002; 167(4):343-8. CPR alone is unlikely to restart the heart, but provision of an electric shock to the subject's heart (defibrillation) is usually needed in order to restore a viable or “perfusing” heart rhythm.
As to morbidity, about 50% of persons that had cardiac arrest for 5 to 8 minutes will suffer from brain damage after successful revival, according to Guyton & Hall, “Textbook of Medical Physiology”, 11th edition, Elsevier Saunders, 2006, page 155, ISBN-13: 978-0-7216-0240-0. CPR is only likely to be effective if commenced within 6 minutes after the heart stops beating because permanent brain cell damage occurs when fresh blood infuses the cells after that time. The cells of the brain become dormant in as little as 4-6 minutes in an oxygen deprived environment, and the cells are unable to survive the reintroduction of oxygen in a traditional resuscitation. In summary, fast detection and early cardio-pulmonary resuscitation (CPR) followed by defibrillation is crucial for a successful outcome.
In many dialysis clinics, the staff cannot provide constant supervision of their patients from this point of view. According to a survey conducted with 385 nurses from 39 countries, as reported by E Lindley in “Venous Needle Dislodgement Survey Dublin 2010”, EDTNA/ERCA VND Project, presentation at EDTNA/ERCA 2011, 58% of the respondents claimed that there are patients in their clinics who are not clearly visible from the nurse's station. 70% stated that the patients are regularly checked, but at long intervals, usually of 30 to 60 minutes. It can be concluded that there is a high risk for cardiac arrest events to pass unnoticed for such a long time that there is no reasonable likelihood of saving the patient from damage or death.
For economic and practical reasons it is undesirable to connect all dialysis patients to dedicated equipment for detecting and signaling cardiac arrest, such as a pulse watch or an electrocardiograph (ECG).
Even if it has been known for a long time to monitor heart pulses in the pressure signal from a venous pressure sensor in a dialysis machine for the purpose of detecting VND, no one has so far suggested detecting and signaling cardiac arrest based on a pressure signal from a pressure sensor in the dialysis machine. It has been suggested to monitor the heart rate in conjunction with VND, e.g. in EP0330761, US2005/0010118 and WO2009/156175, but this does not imply that the heart rate is used or even may be used for the purpose of detecting cardiac arrest. It should be understood that outputting a signal that truly represents the heart rate is not a trivial task, especially when the heart rate disappears or the heart pulses become very weak. During dialysis treatment, the blood pump is running and creates strong pulsations in the pressure signal, especially if the blood pump is of the normal, peristaltic type. It is not uncommon for the pulsations from the pump to be much stronger than the pulsations from the heart in the pressure signal. Even if filtering may be employed for suppressing the pulsations from the pump, such filtering is normally incomplete, leaving at least weak residuals of the pulsations from the pump in the filtered pressure signal. This means that if the heart stops, the algorithm or circuitry for extracting the heart rate is likely to identify the residuals and still output a frequency signal in the region of a normal heart rate (since the blood pump is normally running at a frequency within the frequency range of heartbeats). All in all, this means that it cannot be surmised that the prior art implies a technique for detecting cardiac arrest given a mere reference to an ability to detect the heart rate in a pressure signal.
Clearly, there is a long-felt but unmet need in the field of hemodialysis for a simple and cost-effective technique of on-line monitoring for cardiac arrest in dialysis patients during dialysis treatment. This need has been known at least since the late 1990s, and while it was suggested already in 1988, in aforesaid EP0330761, to monitor the heart rate using the pressure signal from a pressure sensor in an EC circuit, no one has made the connection that cardiac arrest could be monitored via such a pressure signal.